Surgical cannulas are well known in the art. Such devices generally include tube-like members that are inserted into openings made in the body so as to line the openings and maintain them against closure. Surgical cannulae can be used for a wide variety of purposes, and their particular construction tends to vary accordingly (see, e.g., U.S. Pat. No. 5,911,714). Flexible endoscopes, endovascular catheters and guidewires, and trocar cannulae such as those used in laparascopic surgery, are examples of such devices. Several U.S. patents recite such devices. See, for example, U.S. Pat. Nos. 5,482,029; 5,681,260; 5,766,163; 5,820,623; 5,921,915; 5,976,074; 5,976,146; 6,007,519; 6,071,234; and 6,206,872.
All of these devices are in use in one form or another and they are helpful to some extent, but they also pose several problems. Flexible endoscopes and endovascular catheters rely on reaction forces generated by pushing against the tissue of the body cavity being explored to navigate around corners or bends in the anatomy. This approach works reasonably well for small-diameter endovascular catheters that are typically run through arteries well supported by surrounding tissue. In this case the tissue is effectively stiffer than the catheter or guidewire and is able to deflect the catheter's path upon advancement into the vessel. The approach is much less successful in the case of flexible endoscopes being guided through a patient's colon or stomach. In these cases the endoscope is either significantly stiffer than the body cavity tissue it is being guided through or, as is the case for the stomach or an insufflated abdomen, the body cavity is sufficiently spacious that the endoscope has no walls at all to guide it. In the case of colonoscopy, the endoscope forces the anatomy to take painful, unnatural shapes. Often, the endoscope buckles and forms “loops” when the colonoscopist attempts to traverse tight corners. Pushing on the end of the flexible endoscope tends to grow the loop rather than advance the endoscope. “Pushing through the loop” relies on the colon to absorb potentially damaging shapes of force to advance the endoscope. In cases of unusually tortuous anatomy, the endoscope may not reach its intended target at all, leaving the patient at risk of undiagnosed and potentially cancerous polyps.
Endovascular catheters have drawbacks as well. While generally flexible enough to avoid seriously damaging the vessel's endothelial surface, guidewires are difficult to guide into small side branches of large vessels such as the coronary ostia or into relatively small vessels connecting to relatively large chambers such as the pulmonary veins. Catheters are even more limited in their ability to deal with greatly tortuous vessel anatomy such as the vessels radiating from the brain's so-called Circle of Willis.
U.S. Pat. No. 5,251,611 (Zehel) discloses the use of one or more rigidizable conduits for conducting exploratory procedures. A particular rigidizable conduit disclosed by Zehel includes a series of nested segments that are traversed by a plurality of cables or wires. Tension is applied to the wires to press the segments together, thereby producing a stiffened conduit. A side effect of the compression reaction to the applied tension is a shortening of the conduit as the segments are pressed together. For procedures involving an instrument that is inserted in the conduit, the shortening of the conduit will result in a change in the relative position of the instrument and the conduit. This change in relative position may have an adverse effect on the placement or use of the instrument.
Ablation and EKG mapping catheters used in cardiological electrophysiology find their intended targets chiefly by trial and error insertion and twisting of a guidewire/catheter accompanied by gross motions of the entire catheter. A need, therefore exists for a cannula system that provides access port for insertion and removal of diagnostic, surgical, or interventional instruments to and from a site within the body to which the physician does not have line-of-sight access. Furthermore, there is a need for cannula systems that can follow a tortuous path through hollow soft-tissue structures without relying on the surrounding tissue to mechanically support and guide its insertion and may be steered and advanced directly to an anatomical point of interest. There is also a need for a cannula system that can accommodate shortening in the cannula and maintain a relative position between the cannula and an instrument inserted therein as well as between the cannula and a target site within the body.